Filter composite materials are known in the prior art and used in the manufacture of flat filter elements, wound filters and the like. Conventional filter systems for wastewater cleaning include flat filter elements in a spaced-apart parallel arrangement. The flat filter elements are embodified as cushions or cassettes in which a drainage structure formed as flexible woven fabric or rigid plate is surrounded on both sides by filtration membranes. The filtration membrane is typically formed as a two-layered composite structure formed from a support nonwoven and a porous membrane layer. The regions between adjacent flat filter elements form channels for a liquid to be filtered, which in accordance with the principle of cross flow filtration flows parallel to the surface of the flat filter elements and hence perpendicularly to the filtration direction.
EP 0 730 490 B1 (whose United States equivalent is U. S. Pat. No. 5,804,280) discloses a filter composite structure comprising a porous filter medium, a porous support medium, and a substrate including a drainage mechanism; and also a method of forming the composite structure. The porous filter medium and the porous support medium are solvent bonded to the substrate in such a manner that the permeability of the filter medium is not substantially decreased. In preferred embodiments, the substrate consists of a polymeric material and includes grooves and the region between adjacent grooves is adhered to the porous support medium. In a further embodiment, the substrate is formed as a sheet or plate having opposing planar surfaces and each planar surface is solvent adhered to a porous support medium and a porous filter medium. The composite structure is formed by a method comprising the following steps:                positioning the porous filter medium, the porous support medium and the substrate on top of each other;        introducing a bonding composition which merely dissolves the substrate slightly and flows into the porous support medium and the porous filter medium, the dissolved substrate being introduced in the process solidifying after removal of the bonding composition and bonding the three layers together.        
DE 37 12 872 A1 describes filter elements composed of a membrane and a through-flowable drainage structure. The drainage structure consists of a woven fabric, a nonwoven fabric, a perforate or embossed foil, or a combination of these layer materials. The essentially flat filter elements of n-angular or round shape include an aperture for fluid conduction and are adhered or welded at their edges and around the aperture in a leakproof manner. In particular embodiments, the membrane is one- or both-sidedly area-bonded, in the form of a laminate, to a woven fabric and/or a nonwoven fabric. The woven/nonwoven fabric therein is in each case bonded to one membrane only.
German utility model DE 20 2005 012 047 U1 discloses a two- or more-layered composite filter medium for removing particles from a fluid stream, comprising a membrane filtration layer and at least one upstream depth filtration layer. Optionally, the composite filter medium may comprise a supporting layer disposed upstream or downstream of the membrane filtration layer. Optionally, the supporting layer may be laminated with the membrane. Preferably, the depth filtration layer, the membrane filtration layer and the optional supporting layer consist of polymeric melt-blown polymer fiber woven fabric, of expanded PTFE (ePTFE) membrane filtration medium and of spun bonded nonwoven, respectively. The optional support layer is in each case only bonded to one membrane filtration layer.
EP 1 554 028 B1 teaches a filter element with multi-layered pleat support. The filter element comprises an upstream pleat support, a filter medium, a multilayered downstream support including a first downstream support layer and a second downstream support layer. The filter medium is typically a microporous filter medium having a pore size of about 0.1 μm to about 10 μm and consisting of conventional filter materials such as, for example, expanded Teflon, nylon, polyether sulfone, polyvinylidene difluoride and the like. The support layers are preferably fabricated from polymeric non-woven fibrous materials, and the first support layer may be laminated to the filter medium. Lamination can be carried out as per conventional laminating techniques known in the prior art.
EP 0 417 287 B1 (whose United States equivalent is U. S. Pat. No. 5,112,487) describes a porous, heterogeneous membrane consisting of a phenylenesulfide-based copolymer and laminated on a polymeric woven or nonwoven fabric.
When a filter system is in operation, particles having diameters too large to pass through the pores of the membrane layer are retained on the membrane surface and some of them remain attached thereto. Such particles accumulate over prolonged periods and build up to form filter cake which increasingly blinds the membrane surfaces and reduces the filtration performance of the system. As part of the equipment maintenance service, the surfaces of the filtration membranes are periodically cleaned mechanically and/or chemically and freed of filter cake, for example by means of brushing, water jet and cleaning solutions. In addition to these inconvenient and costly cleaning methods, which generally necessitate the deinstallation of the filter elements, an in situ clean by means of backflushing is a possibility. In backflushing, the filter elements are briefly operated, not with underpressure, but with an increased inner pressure such that liquid will flow from the interior of the filter element through the filtration membranes to the outside and detach particles attaching to the surface of the filtration membranes. This backflush is done periodically during ongoing operation, the period interval and the ratio of filtration time to backflush time depending on the current filtration conditions with the period interval typically being between 1 and 300 min, preferably between 5 and 100 min and more preferably between 8 and 30 min. The cleaning effect of backflushing mainly depends on the force acting on attached particles. This force is a function of the internal pressure in the filter element. The increased internal pressure may cause damage to the filter element in that increased internal pressure frequently causes cracks in and delaminations of the filtration membrane. Delamination can occur within a filtration membrane between the support nonwoven and the porous membrane layer, or between the filtration membrane and the drainage structure. As a consequence of delamination, the filtration membrane or the porous membrane layer will frequently inflate to such a degree that it will press against an adjacent filter element, causing the backflush to completely cease at the points affected and the attached filter cake being to some extent pressed into the membrane surfaces.
To avoid such trouble and damage, the internal pressure in backflushing is typically limited to values below 0.05 bar. Increasing the internal pressure to values above 0.05 bar would improve the effectiveness of cleaning by means of backflushing and lengthen the intervals between the costly and inconvenient chemo-mechanical cleans.